Charybdis (singular and plural) appear to be gigantic maws surrounded by fins or flippers, attached to a bladder-like body. They live in shallow waters, where they wait for ships to pass by. When a ship passes, they suck in water, creating a whirlpool. The whirlpool causes 3d6 points of hull damage to ships, and similar damage to any creatures unfortunate enough to be in the water at the time. The following round, the Charybdis spews for the the water, again damaging the vessel. Swimmers and sailors may save vs. Dragon Breath to take half damage from the expectoration, but not from the whirlpool intake. Charybdis then repeat this attack pattern every two rounds until the ship is destroyed. Charybdis are only damaged by magical weapons or spells.

Cool! I am currently reading Avram Davidson's The mirror and the phoenix and one of the characters just mentioned the terrible "Cherub Dys," who lives in a cave over the pillars of Hercules and watches passerbys, killing those it disapproves of. A nice variant.